Gracefully handle when assets are not configured
Gracefully handle when assets are not configured

In sprockets-rails v3, app.assets is now only set when assets.compile is
set to true.  This is currently causing an error when upgrading to Rails
4.2.5.

This change will cause CSS Splitter to only be registered when assets
themselves are configured.

Fixes #60

zweilove/css_splitter,zweilove/css_splitter